1. Field of the Invention
The present invention relates to humidification of a fuel cell, and in particular, to a structure and method for humidifying a fuel cell wherein humid exhaust air from the cell is exposed to one side of a humidification membrane whose other side is on contact with the flow of inlet air into the cell. Contact between the inlet air flow and the humidification membrane moistens the inlet air flow, thereby preventing dehydration of the fuel cell.
2. Description of the Related Art
The present invention relates to electrochemical fuel cells and a method of operating electrochemical fuel cells. A fuel cell generates electricity by carefully directing the flow of electrons involved in the catalyzed reaction of hydrogen and oxygen to form water. Electrochemical fuel cells convert fuel and an oxidant to electricity and a reaction product. A typical fuel cell consists of a cathode, an anode, and an electrolyte. The electrolyte is sandwiched between the cathode and anode. Fuel, in the form of hydrogen, is supplied to the anode where a catalyst (usually platinum) catalyzes the following reaction:
Anode reaction: H2xe2x86x922H++2exe2x88x92
Hydrogen separates into hydrogen cations and electrons. The cations (protons) migrate through the electrolyte membrane to the cathode. The electrons migrate via an external circuit in the form of electricity.
An oxidant, typically oxygen or oxygen-containing air, is supplied to the cathode where it reacts with hydrogen cations that have crossed the proton exchange membrane and electrons from an external circuit. This reaction produces water and is also usually catalyzed by platinum and occurs as follows:
Cathode reaction: xc2xdO2+2H++2exe2x88x92xe2x86x92H2O 
Thus the fuel cell generates both electricity and water through the electrochemical reaction.
While some types of fuel cells employ a liquid electrolyte, the proton exchange membrane (PEM) fuel cell relies on a polymeric membrane to serve as its electrolyte. When hydrated, the polymeric proton exchange membrane possesses acidic properties which allow the membrane to conduct protons from the anode to the cathode of the fuel cell. However, if the proton exchange membrane is not sufficiently humidified, its resistance to the flow of protons increases, the electrochemical reaction occurring in the fuel cell can no longer be supported at a sufficient state, and the output current decreases or, in the worst case, stops.
If a PEM fuel cell is designed to draw its oxygen from a flow of oxygen-containing air, the inlet air flow rate will generally evaporate water from the region containing the proton exchange membrane more quickly than water is generated by the fuel cell. For this reason, PEM fuel cells commonly incorporate an element to humidify the incoming air stream.
A number of fuel cell humidification structures and techniques have been proposed.
U.S. Pat. No. 5,879,826 is entitled xe2x80x9cProton Exchange Membrane Fuel Cellxe2x80x9d (xe2x80x9cthe ""826 Patentxe2x80x9d) and issued on Mar. 9, 1999. The ""826 Patent describes a PEM fuel cell design featuring a humidification manifold. The ""826 Patent is hereby incorporated by reference.
The technique of the ""826 Patent is to flow inlet air across a first side of a humidification membrane, with the second side of the membrane exposed to a flow of liquid water. The humidification membrane is impermeable to the liquid water flow, but is continually rehydrated by contact with liquid water on the second side as the continuous flow of dry inlet air evaporates water from the first side.
Because heat is also generated during the electrochemical reaction within the fuel cell, most fuel cells also incorporate a cooling element to maintain temperature within a range optimum for efficiency. In the fuel cell described by ""826 Patent, the same liquid water supply is utilized both for humidification and cooling. Thus separate structures for humidification and cooling are not required. Another advantage is that humidification of the fuel cell takes place at approximately the operating temperature of the fuel cell.
While the approach of the ""826 Patent works well, it possesses some disadvantages.
One disadvantage is that coupling of humidification and cooling functions precludes protection against freezing. This is because exposure of the humidification membrane to an ethylene glycol antifreeze mixture could degrade the relatively thin and fragile humidification membrane, or could allow constituents of the anti-freeze to be carried into and thereby poison the power section of the fuel cell.
Therefore, there is a need in the art for a humidified fuel cell which incorporates freeze protection.
A second disadvantage of the approach of the ""826 Patent is the complexity of the resulting fuel cell structure. The fuel cell of the ""826 Patent requires a flow of liquid water into the device. Such a liquid water flow may be unnecessary if cooling is provided by air rather than by water. Therefore, there is a need in the art for a humidified, air-cooled fuel cell.
A third disadvantage of the ""826 Patent is the necessity of sealing the humidification manifold against the leakage of liquid water. This is necessary because the liquid water is introduced into the fuel cell under pressure in order to provide for constant circulation of coolant. The need to seal the humidification manifold further complicates design and fabrication of the fuel cell. Therefore, there is a need in the art for a humidified, air-cooled PEM fuel cell having a simple and rugged structure.
Another approach to fuel cell humidification is set forth in U.S. Pat. No. 5,853,910, entitled xe2x80x9cFUEL CELL POWER GENERATING APPARATUS AND OPERATION METHOD THEREFORxe2x80x9d (xe2x80x9cthe ""910 Patentxe2x80x9d), which issued to Tomioka et al. on Dec. 29, 1998. During operation of the fuel cell proposed by the ""910 Patent, moisture-containing exhaust of the fuel cell is conveyed through an air discharge passage and recirculated through the fuel cell via a circulation passage and an air introduction passage. In this manner, the fuel cell exhaust is reintroduced directly to the inlet air flow, thereby preventing dehydration of the proton exchange membrane with moisture present in the exhaust.
The ""910 Patent also possesses some important disadvantages.
One disadvantage is that the humidification approach of the ""910 Patent will adversely impact the power output of the fuel cell. Specifically, the purpose of the fuel cell is to generate electricity by causing reaction between hydrogen and oxygen present in the inlet air flow. Because the ""910 Patent proposes to reintroduce oxygen-depleted exhaust directly into the inlet airflow, the design of the ""910 Patent effectively dilutes the flow of oxygen into the fuel cell and thereby limits the maximum power output by the fuel cell.
Therefore, there is a need in the art for a humidified fuel cell whose power output is not adversely affected by the humidification process.
Another disadvantage of the ""910 Patent is that it calls for active regulation of an airflow valve by a central processing unit. Apart from the additional expense required by such a valve and controller apparatus, the actively-regulated humidification system of the fuel cell of the ""910 Patent would require regular calibration and maintenance.
Therefore, there is a need in the art for a humidified fuel cell involving an effective and passive humidification system.
The present invention teaches a structure and method for humidifying a fuel cell wherein moisture-containing exhaust from the fuel cell transfers moisture to a first side of a humidification membrane. The second side of the humidification membrane is in contact with the inlet air flow. The inlet air flow receives water from the humidification membrane, and subsequent exposure of the proton exchange membrane to this humidified inlet air flow prevents dehydration of the proton exchange membrane without adversely affecting the power output of the cell.
A first embodiment of a fuel cell structure in accordance with the present invention comprises an inlet receiving an inlet oxidant gas flow, a humidification membrane including a first side and a second side, and a first humidification plate receiving the inlet oxidant gas flow and passing the inlet oxidant gas flow across the first side of the humidification membrane. A power section receives the inlet oxidant gas flow from the first humidification plate and passes the inlet oxidant gas flow across one side of a proton exchange membrane to create an exhaust gas flow enriched in water. A second humidification plate receives the exhaust gas flow from the power section and passes the exhaust gas flow across the second side of the humidification membrane, such that moisture from the exhaust gas flow is transferred across the humidification membrane to the inlet oxidant gas flow.
A first embodiment of a method for humidifying a fuel cell in accordance with the present invention comprises the steps of contacting an inlet oxidant gas flow with a first side of a humidification membrane, causing an electrochemical reaction to transform the inlet oxidant gas flow into a moisture-containing exhaust gas flow, and contacting the exhaust gas flow with a second side of the humidification membrane, such that moisture from the exhaust gas flow is transferred across the humidification membrane to the inlet oxidant gas flow.
The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.